The field of the invention is nuclear magnetic resonance imaging MRI methods and systems. More particularly, the invention relates to the design and manufacture of multi-element coil arrays for use in MRI systems.
When a substance such as human tissue is subjected to a uniform static magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a time-varying magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals also referred to as “views” are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The NMR signals are detected using an rf antenna in the form of one or more rf coils. MRI systems include a whole-body rf coil that can receive NMR signals emanating from anywhere in a subject being imaged, but it is also common practice to use specially designed local rf coils when imaging specific anatomy. These local coils are positioned very close to the anatomy being imaged and the result is an increased sensitivity to the NMR signals and a consequent higher SNR in the image reconstructed from those signals.
While single element local coils are used in some clinical applications, because of their limited receptivity field it is also common practice to employ multi-element rf coil arrays. Each coil element operates as a separate rf antenna and is connected to a separate receive channel in the MRI system. The separate NMR signals are combined to increase the receptivity field of view to that of the combined rf coil elements.
When using an array of rf coils to receive NMR signals from a subject being examined, there are two design objectives that should be met to maximize coil sensitivity. First, the coil elements in the array should follow as closely as possible the contour of the subject being imaged, and second, the mutual inductance between each rf coil element in the array should be minimized to reduce interaction between receive channels.
Many multi-element rf coil array designs are based on the overlapping coil element design first disclosed in U.S. Pat. No. 4,825,162 issued on Apr. 25, 1989 and entitled “Nuclear Magnetic Resonance “NMR” Imaging With Multiple Surface Coils”. Multiple flat rf coil elements are placed next to one another to cover the desired field of view in the subject and mutual inductance between adjacent coil elements is minimized by carefully overlapping adjacent coil elements a specified amount. Coil elements can be arranged in a row, with each coil element overlapping the next one by the critical amount to form a linear array (such as a spine array). Additional rows of coil elements can be arranged next to each other overlapping in the second dimension to produce a planar array. Such a planar array can be curved into a shape having one dimension of curvature (i.e., curving along only one dimension). For example, a linear or planar array can be wrapped around to form a cylinder. It is easy to maintain the required coil element overlap when the coil array is substantially flat or is curved in only one direction (e.g., cylindrical), but when the anatomy being imaged requires a more complex curvature, it is difficult to maintain the desired coil element overlap. Such complex curved surfaces are referred to herein as surfaces having non-zero Gaussian curvature. This means that the surface curves in all directions from at least one point on the surface.
A number of factors are driving the number of coil elements in rf coil arrays upward. The sensitivity of a single circular receive element achieves the maximum possible sensitivity at a depth equal to the element diameter divided by the square root of 5. A large array of receive elements is able to achieve the maximum possible sensitivity at any depth greater than the single element diameter divided by the square root of 5. By reducing the size of each coil element, increasing the number of substantially planar coil elements and shaping the array to more closely follow the contour of the anatomy being imaged the array can achieve close to the optimum possible sensitivity throughout the volume enclosed by the array. This increases coil sensitivity and image SNR. Another factor is the use of parallel imaging methods such as SENSE (U.S. Pat. No. 6,326,786), and GRAPPA (U.S. Pat. No. 6,841,998). Parallel imaging uses the separate NMR signals from rf coil arrays to reduce the number of views that are required to reconstruct an image. Hence, the larger the number of coil elements and corresponding receive channels used, the shorter the scan time.
The design and manufacture of coil arrays having larger numbers of coil elements and complex curvatures while maintaining minimum mutual inductance between coil elements has become a very challenging task.